CMOS device for use in connection with an active matrix panel

ABSTRACT

An active matrix panel including a matrix of driving electrodes couples through thin film transistor switches to a corresponding source line and gate line and at least one of a driver circuit including complementary thin film transistors for driving the source and/or gate lines of the picture elements on the substrate. The thin film transistors of the active matrix have the same cross-sectional structure as the P-type or the N-type thin film transistors forming the driver circuit and are formed during the same patterning process.

This is a division of application Ser. No. 07/351,758, filed on May 15,1989, entitled active matrix panel, now U.S. Pat. No. 5,250,931.

BACKGROUND OF THE INVENTION

The invention relates generally to an active matrix panel and moreparticularly to a high density active matrix panel formed with thin filmtransistors, for driving a liquid crystal display (LCD).

A conventional active matrix liquid crystal display panel including amatrix of liquid crystal picture elements formed with thin filmtransistors (TFT's) on a transparent substrate is described in Morozumiet al., "Black and White and Color Liquid Crystal Video DisplaysAddressed by Polysilicon TFTs", SID-83 Digest, pp. 156-57 and is shownin FIG. 19. A monocrystalline silicon MOS integrated gate line drivercircuit 4' for driving a plurality of gate lines 4a' and a source linedriver circuit 4 for driving a plurality of orthoganol source lines 4aare formed on a flexible substrate 3. An active matrix panel 1 includesa matrix of liquid crystal picture elements at the cross-over ofrespective gate lines 4a' and source lines 4a and a plurality ofelectrical connection pads 5. Driving circuits 4 and 4' are electricallycoupled to panel 1 at pads 5. Both flexible substrate 3 and panel 1 aremounted on a substrate 6 and integrated driver circuits 4 and 4' areelectrically coupled to other circuitry (not shown).

Such conventional active matrix panels can provide viewable displays,but they can have the following disadvantages.

1. Inadequate resolution.

Flexible substrate 3 and source lines 4a and gates lines 4a' of activematrix panel 1 are electrically coupled at pads 5. Accordingly, thepicture elements cannot be sufficiently densely spaced because of thespace occupied by pads 5. This interferes with mass production of activematrix panels having a picture element pitch of 100 μm or less andprevents high resolution.

2. Inadequate display device miniturization.

Driver integrated circuits 4 and 4' are located outside of panel 1 onsubstrate 6. Accordingly, active matrix panel 1 occupies only about 1/4or 1/5 of the surface area of substrate 6. Consequently, display devicesincluding conventionally formed active matrix panels are undesirablylarger than the picture element matrix portion of the entire panel. Thismakes it inconvenient to include conventional active matrix panels whenminiaturization is needed, such as for a micro-monitor which can be usedas an electric view finder for a video camera.

3. High manufacturing costs.

Manufacturing a conventional display including an active matrix panelrequires many connections as follows. Active matrix panel 1 is connectedto flexible substrate 3; driver integrated circuit 4 is connected toflexible substrate 3; and flexible substrate 3 is mounted on mountingsubstrate 6. These multiple connection steps increase manufacturingcosts.

4. Low reliability.

Because conventional active matrix panels require so many connections,when stress is applied to the panel, these connections can come apart.This affects the reliability of the entire display and increases costsbecause extra measures must be undertaken to compensate for thepossibility of disconnections.

Accordingly, it is desireable to develop an improved active matrix panelwhich does not have the shortcomings of conventional active matrixpanels.

SUMMARY OF THE INVENTION

Generally speaking, in accordance with the invention, an active matrixdevice includes a substrate with a matrix of thin film transistorswitching elements formed thereon. A gate line driver circuit and/or asource line driver circuit includes thin film transistors in acomplementary metal oxide semiconductor (CMOS) configuration formed onthe substrate having the same cross-sectional structure as the switchingelements. The driver circuit thin film transistors are either of the Ptype of N type. The thin film transistors in a CMOS configuration arealso referred to herein as complementary thin film transistors.

In one embodiment, the gate line driver circuit and/or the source linedriver circuit on the panel substrate includes a static shift registerformed of complementary thin film transistors. In another embodiment,the gate line driver circuit and/or the source line driver circuitinclude P-type and N-type thin film transistor in which the P-type thinfilm transistor includes acceptor impurities in the source region anddrain region and the N-type thin film transistor includes donorimpurities having a higher concentration than the acceptor impurities inthe source and drain regions. Alternatively, the P-type thin filmtransistor includes donor impurities and acceptor impurities with ahigher concentration of acceptor impurities than the donor impurities inthe source region and drain region. The gate length of the P-type andN-type thin film transistors forming the gate line and source linedriver circuits is shorter than the gate length of the thin filmtransistors of the active element matrix.

Accordingly, it is an object of the invention to provide an improvedactive matrix panel.

Another object of the invention is to provide an active matrix panelthat is low in price and high in resolution and reliability.

A further object of the invention is to provide an active matrix panelwhich has low active element pitch.

Still another object of the invention is to provide an improved activematrix liquid crystal display panel having a high density of pictureelements.

Still a further object of the invention is to provide an improvedminiaturized active matrix panel.

Yet another object of the invention is to provide an improved activematrix panel that can be used as an electric view finder for a videocamera, a monitor for a portable VCR and the like.

Still other objects and advantages of the invention will in part beobvious and will in part be apparent from the specification anddrawings.

The invention accordingly comprises the several steps and the relationof one or more of such steps with respect to each of the others, and thearticle possessing the features, properties and the relation ofelements, which are exemplified in the following detail disclosure, andthe scope of the invention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the invention, reference is had to thefollowing description taken in connection with the accompanyingdrawings, in which:

FIG. 1 is a block circuit diagram of an active matrix panel constructedin accordance with the invention;

FIGS. 2A, 2B, 2C, 2D, 2E and 2F are circuit diagrams showing details ofthe driver circuits of FIG. 1;

FIG. 3A is a cross-sectional view of a pair of complementary thin filmtransistors of the driver circuits of FIG. 1;

FIG. 3B is a cross-sectional view of a liquid crystal picture element ofa display device including the active matrix panel of FIG. 1;

FIGS. 4A, 4B, 4C and 4D are cross-sectional views illustrating the stepsfor forming the thin film transistors of an active matrix panel inaccordance with the invention;

FIG. 5 is a graph comparing current-voltage characteristics of a TFTformed in accordance with the invention and a conventionalmonocrystalline silicon metal oxide semiconductor field effecttransistor (MOSFET);

FIG. 6 is top plan view illustrating the dimensions of gate length andgate width of a thin film transistor gate formed in accordance with theinvention;

FIG. 7 is a cross-sectional view illustrating dimensions of depletionlayer width and silicon film thickness in a TFT prepared in accordancewith the invention;

FIG. 8 is a top plan view of an active matrix panel arranged inaccordance with the invention showing location of the elements of thedevice;

FIG. 9 is a top plan view of a unit cell of a driver circuit formed inaccordance with the invention;

FIGS. 10A and 10B are top plan views of inverters of thin filmtransistors formed in accordance with the invention;

FIG. 11A is a circuit diagram of a source line driver for an activematrix panel formed in accordance with the invention;

FIG. 11B is a timing diagram for the source line driver circuit shown inFIG. 11A;

FIG. 12 is a circuit diagram of a shiftline register portion of anactive matrix panel formed in accordance with the invention;

FIG. 13A is a circuit diagram of a shiftline register portion of anactive matrix panel formed in accordance with the invention;

FIG. 13B is a timing diagram for the circuit of FIG. 13A;

FIG. 14 is a circuit diagram of an active matrix panel including a shiftregister in a source line driver circuit formed in accordance with theinvention;

FIG. 15A is a schematic circuit diagram of a picture element of anactive matrix panel constructed in accordance with the invention;

FIG. 15B is a cross-sectional view of the picture element illustrated inFIG. 15A;

FIG. 16A is a cross-sectional view illustrating mounting of a liquidcrystal display device constructed in accordance with the invention;

FIG. 16B is a top plan view of the display device of FIG. 16A;

FIG. 17 is a block diagram of an electric view finder including anactive matrix liquid crystal display panel formed in accordance with theinvention;

FIG. 18 is a top plan view of a projection type color device includingan active matrix panel constructed in accordance with the invention; and

FIG. 19 is a plan view of a conventional active matrix panel.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An active matrix panel formed in accordance with the invention is wellsuited for driving a liquid crystal display (LCD). The active matrixpanel includes a plurality of gate lines and source lines and a thinfilm transistor at each intersection coupled to a liquid crystal drivingelectrode all formed on a first transparent panel substrate. A secondtransparent substrate with transparent common electrodes thereon isspaced apart from the panel substrate and a liquid crystal material isplaced in the space between the substrates.

At least one of a gate line driver circuit and a source line drivercircuit is formed on the panel substrate and coupled to the gate linesand source lines, respectively. The driver circuits includecomplementary thin film transistors (TFT's) of thin film silicon ofP-type and N-type. The thin film transistors that form the switchingelements of the picture element matrix are formed with the samecross-sectional structure as those for the driver circuits. The gateline driver circuit and the source line driver circuit can include astatic shift register in the form of a complementary metal oxidesemiconductor (MOS) structure.

The P-type thin film transistors of the driver circuits include acceptorimpurities in the source and drain regions. The N-type thin filmtransistors include acceptor impurities and donor impurities, but have ahigher concentration of donor impurities than acceptor impurities in thesource and drain regions. In an alternative embodiment, the N-type thinfilm transistors include donor impurities in the source and drainregions and the P-type thin film transistors include donor and acceptorimpurities, but have a higher concentration of acceptor impurities thandonor impurities in the source and drain regions. In another embodiment,the length of the gate of the P-type and N-type type thin filmtransistors of the source line and gate line driver circuits are shorterthan the gate regions of the thin film transistors coupled to thedriving electrodes for the picture elements of the active matrixdisplay.

FIG. 1 is a block circuit diagram illustrating the structure of anactive matrix panel 10 constructed and arranged on a transparentsubstrate 11 in accordance with the invention. A source line drivercircuit 12 including a shift register 13; a gate line driver circuit 21including a shift register 20 and a buffer 23 if desired; and a pictureelement active matrix display 22 are formed on transparent substrate 11.Active matrix 22 is of the complementary metal oxide semiconductor(CMOS) structure formed of silicon thin film.

Source line driver circuit 12 includes a plurality of sample and holdcircuits 17, 18 and 19 formed of thin film transistors (TFT's) and aplurality of video signal buses 14, 15 and 16. Gate line driver 21includes a shift register 20 coupled to a buffer 23 for use whenrequired.

Picture element matrix 22 includes a plurality of source lines 26, 27and 28 electrically coupled to source line driver circuit 12 and aplurality of gate lines 24 and 25 electrically coupled to gate linedriver circuit 21. A plurality of picture elements 32 and 33 are formedat intersections of each source line 26, 27, 28, etc. and gate line 24,25, etc. Each picture element 32, 33, etc. includes a TFT 29 coupled toa portion of the liquid crystal panel identified as a liquid crystalcell 30. Liquid crystal cell 30 includes a picture element electrode (94in FIG. 3B) formed on panel substrate 11 and an opposed common electrode31 (97 in FIG. 3B) on the opposed substrate (98 in FIG. 3B) and a liquidcrystal material (96 in FIG. 3B) therebetween. A counter or a decoderfor selecting a source line and a gate line in order can be substitutedfor shift registers 13 and 20.

Active matrix panel 10 is operated by applying a clock signal CLX and astart signal DX to input terminals 34 and 35 of source line drivercircuit 12. A plurality of video signals V₁, V₂, and V₃ are input into aplurality of corresponding input terminals 36 of source line drivercircuit 12. A clock signal CLY and a start signal DY are input into apair of input terminals 37 and 38 of gate line driver circuit 21,respectively.

Shift registers 13 and 20 can be either of the static type or dynamictype circuit formed of complementary P-type and N-type TFT's or of thedynamic or static type circuit of monoconductive type TFT's. However, inview of the performance characteristics of TFT's, the static typecircuit formed of complementary TFT's is preferred.

An active matrix panel is generally formed of polycrystalline oramorphous silicon on an insulating substrate. Such TFT's have smaller ONcurrent and larger OFF current compared to metal oxide semiconductorfield effect transistors (MOSFET's) which is formed of monocrystallinesilicon. This is due to the fact that the trap density existing in asilicon thin film is higher than the trap density in monocrystallinesilicon. In view of this, the carrier mobility is reduced andrecombination of carriers at reversibly biased P-N junctions occursfrequently.

In view of these TFT's performance characteristics and for the followingreasons, it is preferred to include a static shift register ofcomplementary TFT's in an active matrix panel formed in accordance withthe invention.

1. When a TFT has a large OFF current, the operating voltage range,operating frequency range and operating temperature range of a dynamiccircuit formed with that TFT is small.

2. A driver circuit is preferably in the form of a complementary MOSstructure with low current compensation for best utilizing the lowcurrent consumption of an active matrix type liquid crystal panel.

3. The required ON current value can be smaller than in a monoconductiveMOS dynamic shift register.

FIG. 2A is a circuit diagram of a portion of shift registers 13 and 20of FIG. 1. The circuit includes a plurality of inverters 41 and 42, eachformed of a P-type TFT 47 and an N-type TFT 48 shown in FIG. 2B. Thecircuit of FIG. 2A also includes shows a plurality of clock inverters 43and 46, each formed of a pair of P-type TFT's 49 and 50 and a pair ofN-type TFT's 51 and 52 as shown in FIG. 2C. As shown in FIG. 2C a clocksignal CL is input into the gate of N-type TFT 52 and a reversed clocksignal CL is input into the gate of P-type TFT 49. The circuit of FIG.2A further includes a plurality of clock inverters 44 and 45 each formedof a pair of P-type TFT's 53 and 54 and a pair of N-type TFT's 55 and 56as shown in FIG. 2D. Reversed signal CL is input into the gate of N-typeTFT 56 and clock signal CL is input into the gate of P-type TFT 53.

FIG. 2E shows an equivalent analog circuit that includes an inverter 57,an N-type TFT 58 and a P-type TFT 59 which may be substituted for clockinverters 43 and 46 in FIG. 2A. As shown in FIG. 2F, an equivalentanalog circuit that includes an inverter 60, an N-type TFT 61 and aP-type TFT 62 which may be substituted for clock inverters 44 and 45.Inverters 43 and 46 and inverters 44 and 45, as represented by theanalog equivalent circuits of FIGS. 2E and 2F, respectively, aresubstantially the same except for the polarities of the clock signalsapplied to the gates of TFTs 58 and 61 and to the gates of TFTs 59 and62 being reversed.

As has been described it is advantageous to construct a driver circuitof an active matrix panel with a complementary metal oxide semiconductor(CMOS) TFT structure. However, the mere inclusion of complementary TFTintegrated circuits in prior art active matrix panel does not providethe advantages obtained in accordance with the invention. The prior artdevices suffer from the following disadvantages.

1. It is complicated and expensive to form a conventional panel byintegrating both a P-type TFT and an N-type TFT on the same substrate.

2. It is difficult to form a P-type TFT and an N-type TFT havingbalanced characteristics, although this is preferred for forming acomplementary TFT integrated circuit.

3. Conventional P-type TFT's and N-type TFT's do not have sufficientdriving ability to form a driver circuit.

These disadvantages have been solved by forming an active matrix panelin accordance with the invention, which has an improved structure,dimensions and materials.

FIG. 3A is a cross-sectional view of a pair of complementary TFT'sincluded in source line driver circuit 12 and gate line driver circuit21 in FIG. 1. A P-type TFT 99 and an N-type TFT 100 are formed on aninsulating substrate 71 of either glass or quartz crystal. TFT's 99 and100 include a pair of thin silicon film channel regions 73 and 76respectively and a plurality of thin silicon film regions 72, 74, 75 and77 which are to be source and drain regions, all disposed on substrate71. Silicon thin films 72 and 74 are doped with impurities to be P-typesemiconductors. Silicon thin films 75 and 77 are doped with impuritiesto be N-type semiconductors. TFT's 99 and 100 each include respectively,a gate insulating film 78 and 79, a gate electrode 80 and 81, aninsulating layer 82 and 84, conductive lines 83 and a passivation film85 formed thereover.

Insulating layers 78, 79, 82 and 84 can be formed of silicon oxides suchas SiO₂, silicon nitrides and the like. Gate electrodes 80 and 81 can beformed of polycrystalline silicon, metals, metal silicides and the like.Conductive line 83 is formed of a layer of conductive material such as alayer of a metal.

FIG. 3B shows a cross-sectional view of picture element 32, 33, etc. ofactive matrix panel 22. Reference numeral 86 identifies the sameinsulating substrate 71 in FIG. 3A. A picture element electrode 94formed of a transparent conductive film such as ITO (indium tin oxide)is coupled to a picture element TFT 101. Regions 87, 88 and 89 ofsilicon thin film are formed of the same silicon thin film layers asregions 72, 73 and 74 of P-type TFT 99 and regions 75, 76 and 77 ofN-type TFT 100 and form a channel region 88, a source region 87 and adrain region 89, respectively. Regions 87 and 89 are impurity-doped inP-type or in N-type and the compositions of impurities included are thesame as those included in regions 72 and 74 or regions 75 and 77.

A gate insulating film 90 of the same layer as gate insulating films 78and 79 is disposed on the silicon thin film. A gate electrode 91 of thesame layer as gate electrodes 80 and 81 and an insulation layer 92 ofthe same layer as in insulation film 82 are disposed thereon. Anelectrode line 93 of the same layer as line 83 is coupled to sourceregion 87 and an insulating film 95 of the same layer as insulating film84 are formed across the entire active matrix display region. An opposedcommon electrode 97 is formed on an opposed transparent substrate 98with a liquid crystal material 96 in the space between substrates 86 and98.

The source-drain region, channel region, gate insulation film and gateelectrodes of TFT's 99 and 100 in the driver circuits are formed of thesame thin film layers of picture element TFT 101. TFT's 99 and 100 ofsource line driver circuit 12 and gate line driver circuit 21 areelectrically connected to the lines of active matrix display 22 throughline layer 83. A source line in display 22 is formed of line layer 93which is the same layer as line 83. Line layer 83 is formed of a metalhaving low sheet resistance, such as aluminum.

When line layer 93 is made of aluminum or alumi-silicide and transparentconductive driving electrode is ITO it is not necessary to dispose aninsulating film therebetween. A pair of through holes 102 and 103 areopened simultaneously to expose source and drain regions 87 and 89 forconnecting conductive line 93 and electrode 94. This simplifies themanufacturing process.

The aluminum and ITO layers are processed in individual etchingsolutions. The ITO is formed prior to the aluminum layer takingadvantage of the fact that the ITO will not soak into the aluminumetching solution.

Insulating film 95 acts as a capacitor for preventing application of DCvoltage to liquid crystal material 96. The capacitive value of thecapacitor should be sufficiently large as compared to the capacitivevalue of the picture element to prevent DC voltage application to liquidcrystal material 96. Thus, the thickness should be set at apredetermined value, for example, about 3,000 Å or less. The drivercircuit portion of panel 10 is covered by passivation film 85 having athickness greater than a predetermined value of about 1 μm to insure awet-proof layer. A preferred method of forming passivation film 85 is toform a film over the entire active matrix substrate and then remove allexcept the driver portions. Accordingly, passivation film 85 ispreferably formed of materials processed with an etching solution suchas polyamide or the like in which insulation films 84 and 85 are notdissolved.

At least four photo processes are required to form a complementary metaloxide semiconductor (CMOS) integrated circuit formed with conventionalmonocrystalline silicon. These steps include forming a low concentrationP well, forming a P-type stopper layer, forming a P-type source anddrain metal oxide semiconductor field effect transistor (MOSFET) andforming a source and drain of N-type MOSFET. However, a complementaryTFT integrated circuit can be formed with as few as one photo processingstep compared with a method of manufacture for monoconductive type TFTintegrated circuits.

FIGS. 4A, 4B, 4C and 4D illustrate steps of forming complementary TFT inan active matrix panel in accordance with the invention. A silicon thinfilm is disposed on a transparent substrate 110 in a desired pattern toprovide silicon thin films 111, 112 and 113 for forming a channel region111' of P-type TFT 132 and channel regions 112' and 113' of N-type TFT's133 and 134. Gate insulating films 114, 115 and 116 are disposed onchannel regions 111, 112 and 113 respectively by thermal oxidation andchemical vapor deposition and gate electrodes 117, 118 and 119 areformed thereon.

As shown in FIG. 4B, acceptor impurities 120, such as boron areimplanted in silicon films 111, 112 and 113 on the surface of substrate110 by ion implantation. Implanted acceptor impurities are activated bysubsequent heat treatment to form P-type semiconductors. At this timeacceptors are present in regions 123, 124, 125 and 126 which will becomethe source and drain regions of N-type TFT's 133 and 134 as well asregions 121 and 122 which become source and drain regions of P-type TFT132.

FIG. 4C shows that P-type TFT 132 is covered with a masking material,such as photo resist 128. Donor impurities 127, such as phosphorous orarsenic are implanted into silicon thin films 112 and 113 at a higherconcentration than acceptor impurities 120 in source and drain regions123', 124', 125' and 126'. Because source and drain regions 121 and 122are covered with photo resist 128, donor impurities 127 do not enterthose regions.

Implanted donor impurities are subsequently activated by heat treatment.If regions 123', 124' 125' and 126' are implanted with a dosage of1×10¹⁵ cm⁻² acceptor ions and implanted with a dosage of 3×10¹⁵ cm⁻²donor ions, these regions are equivalent to regions having a donorconcentration corresponding to an implant dosage of 2×10¹⁵ cm⁻².Accordingly, P-type source region 121 and drain region 122 and N-typesource regions 123' and 125' and drain regions 124' and 126' are formedwith only one masking step. After photo resist 128 is removed, aninsulating layer 129 is disposed over the entire surface of substrate110. A plurality of through holes 129' are formed in insulating film 129and gate insulating films 114, 115 and 116 at each TFT to expose sourceand drain regions 121, 122 and 123'-126'.

A picture element electrode 131 formed of a transparent conductive filmis disposed on insulating layer 129 and is electrically coupled to drainregion 126' at through hole 129'. A plurality of lines 130, formed ofmetal or the like are disposed on insulating layer 129 and areelectrically coupled with source and drain regions 121, 122 and123'-125' through the respective through holes 129' in insulating layer129. P-type TFT 132 and N-type TFT 133 form a complementary TFT drivercircuit portion of an active matrix panel and N type TFT 134 is anactive element for the liquid crystal picture elements.

The above sequence of donor and acceptor impurity implantation can bereversed. The initial implantation can be with donor impurities and thesubsequent implantation with masking over the N-type TFT's can be withacceptor impurities. The P-type TFT would include both donor andacceptor impurities, but would have a higher concentration of acceptorimpurities.

As shown in FIGS. 4A-4D, a complementary TFT integrated circuit can beformed with only one additional photo masking step to form an activematrix panel with a built in driver circuit. This has advantages overmethods for forming monoconductive type TFT integrated circuits whichrequire several additional masking steps. The lower number of maskingsteps has advantages, including lowering production costs. Because eachTFT is electrically separated from the others by insulating layer 129,further steps for separating the TFT's are not required. In addition,problems associated with parasitic MOSFET do not occur because theintegrated circuit is not formed of monocrystalline silicon so that achannel stopper is not required.

It is necessary that the P-type TFT and the N-type TFT of thecomplementary TFT integrated circuit have balanced characteristics. Itis known to make TFT's with group II-VI semiconductors. However,complementary TFT's cannot be formed from these compounds for thefollowing reasons.

1. It has been found not to be possible to control and form both P and Nconductive types in the semiconductor compound.

2. It is difficult to control adequately the interface between thesemiconductor compound and the insulating film for metal oxidesemiconductor (MOS) construction.

Accordingly, source, drain and channel regions of TFT's are preferablyformed of thin silicon films. Carrier mobilities of amorphous siliconthin films and polycrystalline silicon thin films are shown in Table 1.It is evident that polycrystalline thin films are preferable for formingcomplementary TFT integrated circuits, because the P-type and N-typecarrier mobilities are similar so that the characteristics of the P-typeand N-type semiconductors can be well balanced and the current supplyingcapacity of the resulting TFT can be increased.

                  TABLE 1                                                         ______________________________________                                                      Carrier Mobility (cm.sup.2 /V · sec)                   Type of Silicon N type      P type                                            ______________________________________                                        amorphous silicon                                                                             0.1-1       10.sup.-4 -10.sup.-3                              polycrystalline silicon                                                                         5-50       5-50                                             ______________________________________                                    

It is advantageous to elevate the current supplying capacity of a TFT,especially the P-type and N-type TFT's which form the driver circuit.The trap density of a TFT formed from a thin silicon film which is notmonocrystalline silicon is high. Consequently, ON current is small andOFF current is larger than with a monocrystalline silicon MOSFET.

FIG. 5 is a graph comparing the current-voltage characteristics of amonocrystalline silicon MOSFET (curve 140) and a thin silicon film TFT(curve 141). The gate length, gate width and source/drain voltage V_(DS)were the same. The abscissa corresponds to the voltage of the source(V_(GS)) as a reference and the ordinate corresponds to the relativevalue of current between the source and the drain (I_(DS)). FIG. 5demonstrates that since the ON/OFF ratio of the TFT is small, TFT 29 ofpicture element matrix 22 and the TFT's forming driver circuits 12 and21 should be formed with certain dimensions to optimize this ratio.

When an image from a National Television System Committee (NTSC) videosignal is to be displayed, the picture element matrix TFT's shouldsatisfy the following equations within the entire temperature range towhich the active matrix panel will be exposed.

    0.1×C.sub.1 ·ROFF1≧1/60 sec          (1)

    5×C.sub.1 ·RON1≦10 μ sec          (2)

C₁ represents the total capacitance of a picture element. RON1 and ROFF1represent ON resistance and OFF resistance respectively of a TFT.Equation (1) should be satisfied by all of the picture elements of thematrix while in a holding operation (holding condition). If thiscondition is satisfied, 90% or more of the electric charge written intothe capacity of the picture elements can be held over one field. Ifequation (2) is satisfied by all of the picture elements in the matrixwhile in the writing operation (writing condition), 99% or more of thevideo signal can be written in picture elements.

The TFT's forming the driver circuit should satisfy the followingequation over the temperature range to which the active matrix panelwill be exposed.

    k×(C.sub.2 ·RON2+C.sub.3 ·RON3)≦1/2f(3)

C₂ and C₃ represent the capacitances at a junction 442 and a junction443 shown in FIG. 2A. RON2 and RON3 correspond to the resistance ofclock inverter 43 and output resistance of inverter 41, respectively.Symbol f is the clock frequency of a shift register and k is a constant,which has been empirically determined to be from about 1.0 to 2.0. Afterperforming a number of trials, it was determined that RON₂ and RON₃should be about 1/10 or less of RON₁ the ON resistance of the pictureelement TFT, to yield a shift register having a clocked frequency (f) ofabout 2 MHz.

The gate length of the TFT of a driver circuit should be formed as shortas possible, within the limits of the permissible breakdown voltage, toachieve this low output resistance. The TFT which forms sample and holdcircuits 17, 18 and 19 of FIG. 1 permits lower breakdown voltage thanthe TFT which forms shift register 13. Accordingly, the gate length ofthe hold circuit TFT's can be shorter than the gate length of the shiftregister TFT's.

FIG. 6 defines the manner of measuring the dimensions of a TFT. FIG. 6shows a gate electrode 142 on a thin silicon film 143 that forms achannel region. Gate electrode 142 overlaps silicon film 143 and has agate length (L) 144 and a gate width 145. Examples of gate lengths ofTFT's of the active matrix panel are shown below in Table 2.

                  TABLE 2                                                         ______________________________________                                                        Gate length L (μm)                                         TFT Function      P-type TFT  N-type TFT                                      ______________________________________                                        TFT for picture element matrix                                                                              20.0                                            TFT for shift register                                                                          4.0         5.5                                             TFT for sample and hold circuit                                                                             4.5                                             ______________________________________                                    

In order to raise the current supplying capacity of a P-type TFT and anN-type TFT, a thickness t_(si) of the thin silicon film between thesource and drain is made to be smaller than the maximum calculateddepletion layer thickness which extends over the surface of the thinsilicon film. The maximum calculated depletion layer thickness forP-type TFT's (X_(PMAX))and for N-type TFT's (X_(NMAX)) formed of thinsilicon films are represented by the following equations, respectively.

    X.sub.NMAX =(2ε×2 φfP).sup.1/2 ×(q×ND).sup.-1/2                              (4)

    X.sub.PMAX =(2ε×2 φfN).sup.178 ×(q×NA).sup.-1/2                              (5)

wherein: q represents a unit electric charge, ε represents thedielectric constant of a thin silicon film, φfP represents a fermienergy of a P type TFT, φfN represents the fermi energy of an N typeTFT, ND represents equivalent donor density of the thin silicon filmformed between the gate insulating film and the insulating substrate andNA represents equivalent acceptor density of the thin silicon filmformed between the gate insulating film and the insulating substrate.The equivalent donor and acceptor densities are determined by thedensity of donor impurities in a region, the density of acceptorimpurities in a region and the trap density which acts as a donor andacceptor. Thickness t_(si) of P-type and N-type TFT's is preferablyformed smaller than either of X_(PMAX) or X_(NMAX).

FIG. 7 illustrates a TFT 152 formed on an insulating substrate 146 andincludes a region 147, a source region 148, a drain region 149, a gateinsulating film 150 and a gate electrode 151. The maximum calculateddepletion layer thickness X_(iMAX)) (i.e., X_(PMAX) or X_(NMAX)) extendsfrom the boundary between gate insulation film 150 and region 147 intosubstrate 146.

To form an active matrix panel in accordance with this aspect of theinvention:

1. The driver circuit is preferably a static shift register circuitformed of complementary TFT's.

2. Complementary TFT integrated circuits are formed;

3. P-type and N-type semiconductors of the complementary TFT are formedto have well-balanced characteristics; and

4. The TFT is formed to have acceptable driving capabilities.

However, further improvements can be made to form an active matrix panelhaving certain improved qualities.

FIG. 8 is a plan view of an active matrix panel 160 showing thepositioning of elements arranged in a preferred configuration. A sourceline driver circuit 161 (162) is formed at the periphery of activematrix panel 160 which is substantially a square in plan view. Sourcelines from source line driver circuit 161 run between the top and bottomof panel 160. A shift register 163, a buffer 164, a video signal bus 165and a sample hold circuit 166 are arranged from the edge towards thecenter respectively within source line driver circuit 161. A gate linedriver circuit 167 (170) is formed at the left or right edge of panel160 and a shift register 168 and a buffer 169 are arranged from the edgetowards the center within gate line driver circuit 167.

A picture element matrix 171 is formed at the center of active matrixpanel 160 and is electrically coupled with source line driver circuit161 and gate line driver circuit 167. A plurality of input terminals172, 173, 174 and 175 are provided at each corner of panel 160. Signalsare transmitted in directions indicated by a plurality of arrows176-180. By arranging the functional portions of active matrix panel 160as shown in FIG. 8, the limited space can be effectively utilized.

FIG. 9 is a preferred circuit pattern layout for a plurality of unitcells 196, 197 and 198 of a driver circuit having a small pitch,equivalent to a picture element pitch (or twice as large as a pictureelement pitch) to be provided in the source line driver circuit and/orthe gate line driver circuit. Reference numerals 181, 182 and 183correspond to either a single picture element pitch or a double pictureelement pitch in which D represents the length. Forming the drivercircuit with cells in sequence, with D as a cycle, while utilizing thelayout of FIG. 8 will provide effective use of space and enhanceminiturization and picture element density.

The unit cells shown in FIG. 9 include a positive power source line 184and a negative power source line 185; a plurality of silicon thin filmregions 186-191 which form a plurality of P-type TFT source, drain andchannel regions; and a plurality of silicon thin film region 192-195which form a plurality of N-type TFT source, drain and channel regions.The elements of each TFT can be separated by etching silicon thin filmto form islands regardless of their homopolarity and heteropolarity.

If the distance between N-type TFT silicon thin film island 192 andP-type TFT silicon thin film region 187 is denoted "a" and the distancebetween P-type silicon thin film region 187 and 188 is denoted "b",distances a and b can be made approximately equal to each other.Accordingly, the integration in the direction in which a unit cell isrepeated can be increased by arranging alternating islands of P-typeTFT's and N-type TFT's to utilize these characteristics advantageously.

FIGS. 10A and 10B illustrate configurations to increase the integrationof these elements. An inverter formed of complementary TFT's is formedbetween a positive power source line 199 and a negative power sourceline 200. A P-type region 204 and N-type region 205 with a boundary 208therebetween are formed in a thin silicon film. A through hole 201 and athrough hole 202 are provided for electrically coupling P-type region204 with positive source line 109 and N-type region 205 with negativesource line 200, respectively. A gate electrode 203 is provided overboth portions 204 and 205. A through hole 206 is provided at the drainportion of regions 204 and 205 to electrically couple an output line 207of the inverter. It is evident that the configuration shown in FIG. 10Bis an effective utilization of space.

It is preferable to reduce the clock noise at source line driver circuit12. As shown in FIG. 1, source line driver circuit 12 is provided withvideo signal buses 14, 15 and 16 and a line for transmitting at least apair of dual clock signals CL and CL for driving shift register 13. Ifthere is a difference between stray capacitance formed between videosignal bus 36 and the CL line and the stray capacitance formed betweenvideo signal bus 36 and the CL line, noise in the form of spikesynchronizing with the clock signal is unintentionally added to thevideo signal. This results in an uneven display and forms lines on thepicture displayed by the active matrix panel.

FIG. 11 is a circuit diagram illustrating a clock line configuration foralleviating this problem. A source line driver circuit including a shiftregister having a plurality of unit cells 210, 211, 212 and 213 isprovided. The unit cells are electrically coupled to a plurality ofsample hold circuits 214 and 215 which are coupled with a pictureelement matrix 216 and a video signal bus 217. A CL line 218 and a CL219 are twisted, crossing near their centers. Accordingly, the averagedistances between CL line 218 and the video signal bus and between CLline 219 and the video signal bus are about equal. As a result, thevalue of stray capacitance (C_(s1) +C_(s3)), which is formed between theCL line and the video signal bus is equal to the value of straycapacitance (C_(s2) +C_(s4)) formed between the CL line and the videosignal bus.

FIG. 11B is a timing diagram for the circuit shown in FIG. 11A. Therising edge of CL corresponds to the trailing edge of CL. The risingedge of CL corresponds to the trailing edge of CL. Consequently, clocknoise added to the video signal is sharply reduced and picture qualityis improved. Similar effects can be achieved by twisting the CL and theCL lines several times.

It is advantageous to provide sample hold circuit lines that have equalresistance. FIG. 12 shows a shift register 230 that is included insource line driver circuit 12 of FIG. 1. Shift register 230 is coupledto a plurality of sample hold circuits 234, 235 and 236 which are alsocoupled to a plurality of video signal buses 231, 232 and 233.Corresponding sample hold circuits 234, 235 and 236 are also coupled toa picture element matrix 240.

Picture element signals corresponding to the colors, red (R), green (G)and blue (B), for example, are transmitted to the three video signalbuses 231, 232 and 233, respectively. The combination is then changed bya single horizontal scanning. Because the three signal buses require lowresistance, it is common to form the signal buses from metals, such asaluminum. However, as has been discussed with reference to thecomplimentary TFT's in FIGS. 3A and 3B, it is advantageous to form theselines from the same material as the gate electrode, which can be formedof polycrystalline silicon. Because the heat resistance ofpolycrystalline silicon thin films is much higher than of metallicfilms, and because the lengths of lines 237, 238 and 239 will not beequal if they are connected in straight lines, the resistances of theselines will not be equal. Differences in line resistance result in unevendisplays and the generation of lines. Accordingly, it is preferable toform lines 237, 238 and 239 so that the resistances will be equal. Thiscan be accomplished by adjusting the widths and lengths of these lines.

It is advantageous to form an active matrix panel with a high speeddriver circuit. However, as shown in FIG. 5, TFT's are generally slowerthan monocrystalline silicon MOSFET's. Accordingly, a conventional shiftregister made from TFT's will not be fast enough to drive an activematrix panel assembled in accordance with the invention. Accordingly,the shift line register circuit shown in FIG. 13A will compensate forthe voltage current characteristics of the TFT's and make up for theirslow speed.

As Shown in FIG. 13A, start signal DX and clocks CLX1 and CLX1 areapplied to a first shift register 250 included in a source line drivercircuit to output sampling pulses 252, 254, etc. Start signal DX andclocks CLX2 and CLX2 are applied to a second shift register 251 includedin the source line driver circuit to output sampling pulses 253, 255,etc. Lines 252-255 are each coupled to a sample hold circuit 256, 257,258 and 259. A video signal bus 265, driven by a signal V, is alsocoupled to sample hold circuits 256-259 which are in turn coupled to aseries of source lines 261, 262, 263 and 264.

The signals and pulses outputed from shift registers 250 and 251 areshown in FIG. 13B. The clocks which drive shift registers 250 and 251have phases that are offset by approximately 90°. When the source linedriver circuit is provided with N system shift resisters, each shiftresister is driven by N system clocks and reverse clocks with phase offset by approximately 180°/N. If the frequency of CLX1 an CLX2 is denotedas f, sampling pulses 252 to 255 are outputed in order by intervals of1/4f hour. Video signal V is sampled at each edge 266, 267, 268 and 269and is held at source line 261 and 264. This results in a sampling witha frequency of 4f. This allows a shift resister driven by a clock offrequency f which makes up for the inherent slow speed of TFT shiftresisters.

When the above described source line driver circuit of FIG. 13A isprovided with N system shift registers, a sampling frequency of 2Nf canbe achieved with a shift register driven by a clock of frequency f.Accordingly, the active matrix panel can be adequately driven by adriver circuit formed of TFT's.

FIG. 14 illustrates an embodiment of the invention in which a testmechanism is provided at each output from source line driver circuit 12and gate line driver circuit 21. Source line driver circuit 12 includesa shift register 280 coupled to a sample hold circuit 282 which iscoupled to a video signal bus terminal 281 by a video signal bus. Samplehold circuit 282 is coupled to a source line driver test circuit 283which is coupled to a control terminal 284, a test signal outputterminal 285 and a source line 286. A gate line driver circuit includesa shift register 287 coupled to a gate line driver test circuit 288which is coupled to a test signal output terminal 290, a gate line 291and a test signal input terminal 289. Gate line 291 and source line 286are coupled to the picture element TFT in display matrix 292 and thetest circuits are coupled to each source and gate line.

A predetermined test signal is input into video signal bus terminal 281and shift register 280 is scanned. If the signal output serially atterminal 285 meets a predetermined standard, it is designated "good",and if not, it is designated "poor". A predetermined test signal isinput and shift register 287 is scanned. If the signal output seriallyat terminal 290 meets a predetermined standard, the gate line drivercircuit is designated "good", and if not, it is designated "poor". Inthis manner, the active matrix panel can be automatically andelectrically tested. Such testing is superior to conventional visualobservations.

It is advantageous to form storage capacitors at each picture elementwithout adding additional steps to the active matrix formationprocedure. FIG. 15A illustrates the equivalent circuit of a pictureelement 327 shown in cross-sectional view in FIG. 15B. The circuit foreach picture element includes a source line 300 and a gate line 301coupled to a picture element TFT 302 which operates as a switch. TFT 302is coupled to a metal oxide semiconductor (MOS) capacitor 305 and aliquid crystal cell 303 including a common electrode 304 and a gateelectrode 306.

Additional details of picture element 327 are shown in FIG. 15B. Pictureelement 327 includes transparent insulating substrates 310 and 324,silicon thin film layer 307 which includes channel regions 312 and 314and doped regions 311, 313 and 315 forming channel and source and drainregions. Gate insulating films 316 and 317 are formed from silicon thinfilm 307 and gate electrodes 318 and 319 formed thereon. An insulatinglayer 320, is formed across the substrate and a source line 321, atransparent conductive film 322 which forms the picture elementelectrode is formed on insulating layer 320. A common electrode 323formed of a transparent conductive film is formed on substrate 324 andliquid crystal material 325 is in the space between substrates 310 and324.

As shown in FIG. 15B, MOS capacitor 305 has the same cross-sectionalstructure as picture element TFT 302. Accordingly, it is not necessaryto add additional manufacturing steps to form MOS capacitor 305, whichcan be formed from the same layers of material as TFT 302 during thesame patterning procedure.

If MOS capacitor 305 is used as a storage capacitor, it should maintaina channel (inversion) layer at region 314. A predetermined voltage isapplied to gate electrode 306 of MOS capacitor 305 to turn capacitor 305ON to maintain inversion layer 314. This can be accomplished with apositive power source for an N-type MOS capacitor or a negative powersource for P-type MOS capacitor.

A gate insulating film is normally extremely thin. Therefore, it canform a storage capacitor that is from 5 to 10 times as large as acapacitor formed with a conventional insulating layer having the samesurface area. Accordingly, the surface area of the capacitor can bereduced to increase the aperture ratio of the active matrix panel.

FIGS. 16A and 16B show advantageous structures for mounting an activematrix panel having a built-in driver circuit in a device. A pictureelement matrix and driver circuit including TFT's having the samecross-sectional structure are formed on a transparent substrate 330. Acommon electrode is formed on an opposed transparent substrate 331 and asealing member 334 fixes the substrates in cooperating relationship. Thegap between the substrates is filled with a liquid crystal material 333.Substrate 330 is disposed in a concave portion 336 of a mountingsubstrate 335 having an aperture 340. A wire 338 formed of a metal suchas gold or aluminum and a protecting member 339 secure substrate 330 inconcave portion 336. Concave portion 336 improves the connectingstrength of wire 338. It is advantageous to provide a shading member 337over a portion of mounting substrate 335 and as a "belt" around theperiphery of opposing substrate 331 to improve the external appearanceof a display device formed of this active matrix panel.

FIG. 16B is a plan view of the mounted panel shown in FIG. 16A. FIG. 16Billustrates the positioning of a picture element matrix portion 341 anda dotted lined 342 illustrates the aperture portion of mountingsubstrate 335.

An active matrix formed in this manner has the following advantages.Stress applied to metallic wires 338 is uniform which improves theconnecting strength. When the active matrix panel is used as a backlittransmissive type display device, unintentional leakage of light aroundthe periphery of the picture element is prevented. An active matrixformed in accordance with invention is also particularly well suited tobe included in an electric view finder (EVF) of a video camera or thelike. By integrating the driver circuit formed of complementary TFT's atthe periphery of the picture element matrix, a small sized, inexpensiveand reliable active matrix panel having low power consumption and highresolution is obtained. A block diagram of a device including anelectric view finder 353 is showing in FIG. 17. A sensing device 350transmits a signal to a video signal processing circuit 351. Circuit 351transmits a signal to a recording apparatus 352 and a composite videosignal to electric view finder 353.

Electric view finder 353 includes a driving circuit portion 354 thatincludes a chroma circuit, a synchronized timing signal formationcircuit, a liquid crystal panel driving signal formation circuit, apower source circuit and a back light driving circuit. Electric viewfinder 353 further includes a luminous source 356 for providing backlight, a reflector 355, a diffuser 357, a plurality of polarizers 358and 360, an active matrix panel 359 and a lens 361. Electric view finder353 has the following advantages over conventional cathode ray tube viewfinders.

1. A color electric view finder of extremely high resolution having apicture element pitch of 50 μm and less can be achieved by including anactive matrix panel having a color filter.

2. Electric view finder 353 uses less power than a cathode ray tube viewfinder.

3. Electric view finder 353 can be smaller and thereby save space.

4. The shape and configuration of electric view finder 353 is moreadaptable and flexible than a CRT view finder, permitting novel designssuch as flat electric view finders.

An active matrix panel constructed in accordance with the invention isadvantageously included in a color projection display device. FIG. 18 isa block diagram of a projection type color display device 390. Projector390 includes a light source 370 such as a halogen lamp focused by aparabolic mirror 371 and an infrared filter 372 for shielding heatgenerated by light source 370 so that only visible rays exit filter 370and enter the dichroic mirror system. A first dichroic mirror 373reflects blue light having a wave length of about 500 nm. Remaininglight is transmitted therethrough. The reflected blue light is reflectedby a reflection mirror 374 and then enters blue light modulation liquidcrystal light valve 378. Light transmitted through dichroic mirror 373illuminates a green light reflecting dichroic mirror 375 and green lighthaving a wave length of about 300 to 600 nm is reflected into a greenlight modulation liquid crystal light valve 379. The remaining light,having a wave length of about 600 nm or longer (red) is transmittedthrough a dichroic mirror 375 and is reflected by a pair of reflectionmirrors 376 and 377 into a red light modulation light valve 380.

Blue, green and red light valves 378, 379 and 380 are active matrixpanels driven by a primary color signal. The blue, green and red lightis synthesized by a dichroic prism system 383. Prism system 383 isconstructed so that the blue reflection surface 381 and a red reflectionsurface 382 cross at right angles. The synthesized color image isprojected and magnified through a projection lens 384.

A projection device including an active matrix liquid crystal displaypanel constructed in accordance with the invention has the followingadvantages over conventional cathode ray tube video projection systems:

1. Projection lens 384 can have a small aperture because the lightmodulating panels can be small and of higher density than a CRT. Thiscan lead to a small, light and inexpensive projection device.

2. Because the active matrix panel has a high aperture ratio, a brightprojection beam can be generated even if the projection lens has a smallaperture.

3. The registration of the red, blue and green colors is excellentbecause the optical axis of the three panels is conformed by thedichroic mirrors and prisms.

By integrating a gate line and a source line driver circuit formed ofcomplementary TFT's on a transparent substrate of a picture elementmatrix, the following advantages can be obtained.

1. Although the degree of resolution in prior art panels is limited by amounting pitch of the driver integrated circuit, by employing the builtin driver integrated circuit in accordance with the invention, a liquidcrystal panel having a picture element pitch of 50 μm and less can beachieved.

2. Because the external dimensions of the mounting substrate can bereduced, the display and the device including the display can besmaller, thinner and lighter.

3. Because it is unnecessary to attach the driver integrated circuit toan external portion, fewer connections are required which lowers thecost of a display device including the liquid crystal panel.

4. Because an external connection for the driver integrated circuit isnot required, the reliability of the display device is improved.

5. By forming the driver circuit with complementary TFT's, the powerrequirements of the device are reduced.

An active matrix panel having these advantages is particularly wellsuited for inclusion in an electric view finder for a video camera, aportable image monitor and a small video projection system.

The active matrix panel will also operate over an extended voltage andoperating frequency range by using complementary TFT's and a circuitstructure with a static shift register. A TFT has a high OFF current andthe temperature dependency of OFF current is also large. However, thesecharacteristics are controlled and compensated for by including a staticshift register which expands the voltage and frequency range.

Because the active matrix can be formed in which first doping impuritiesare included in the TFT source and drain regions and then second dopingimpurities are included having opposite polarity and a higherconcentration than the first impurities, an inexpensive complementaryTFT integrated circuit can be obtained with only one additional photoprocess and P type and N type TFT's having well balanced performance canbe conveniently obtained.

The length of the gate of the TFT's which form the driver circuit isshorter than the gate of the TFT's which form the picture elements. Thisallows the actuating speed of the driver circuit to be increased and thewriting and holding of electric charge of each picture element can beoptimized.

The following features can be included in an active matrix formed inaccordance with the invention. The integration of the driver circuitportion is increased by the pattern layout of functional blocks shown inFIGS. 8, 9, 10a and 10b, so that the unit cells can be formed within asmall pitch such as the picture element pitch. The clock noise which canunintentionally mingle with video signals can be removed to improve thedisplay image. The resistance of the connection lines to the sample holdcircuits are made uniform so that the writing level of the displaysignal to all of the source lines is made uniform which improves displaycharacteristics.

Further advantages are achieved when a source line driver circuit isformed as shown in FIG. 13A and is driven by the method shown in FIG.13B which includes N series shift registers driven by a clock offrequency f so that the video signal can be sampled with a frequency of2Nf. This allows use of a built in driver circuit including TFT's whoseON current is not necessarily large enough.

Including a test circuit in each output of the driver circuit allowschecking of an active matrix panel. Previously this is carried out byvisual examination of a conventional test patter. Now this can becarried out electrically and automatically. Provision of a storagecapacitor in each picture element as shown in FIGS. 15A and 15B permitthe electric charge in each element to be held more steadily. This isdone at no increase in cost of production or decrease in aperture ratio.

The mounting structure of FIGS. 16A and 16B also prevents unintentionalleakage of light around the periphery of the picture element portions ofthe matrix. This improves performance of back lit devices as well astransparent display devices. The advantages of an active matrix panelformed in accordance with the invention permits the construction ofelectric view finders that are superior to conventional cathode ray tube(CRT) view finders. By employing an active matrix panel with a pictureelement pitch of 50 μm or less and a color filter, extremely highresolution color electric view finders can be formed. These view finderswill have low power consumption, small size and light weight. They canbe included in novel designs such as flat electric view finders.

Projection type color display devices including active matrix panelsconstructed in accordance with the invention have advantages not foundin conventional CRT projection devices. The image can be formed on apanel that is smaller and has higher resolution than a CRT, a smalleraperture projection lens can be used and a smaller, lighter and lessexpensive projection device can be provided. Because of the highaperture ratio of the active matrix panel, a bright display can beobtained with a small aperture projection lens. The optical axis of thered, green and blue light valves will completely coincide due to theeffects of the dichroic mirrors and dichroic prisms so that registrationof the three colors can be performed satisfactorily.

It will thus be seen that the objects set forth above, among those madeapparent from the preceding description, are efficiently attained and,since certain changes may be made in carrying out the above method andin the article set forth without departing from the spirit and scope ofthe invention, it is intended that all matter contained in the abovedescription and shown in the accompanying drawings shall be interpretedas illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed and all statements of the scope of the invention which, as amatter of language, might be said to fall therebetween.

What is claimed is:
 1. A CMOS device, comprising:a substrate; a thinsilicon film carried by said substrate; a thin film transistor formedfrom said thin silicon film, and carried by the substrate, said thinfilm transistor having a source region and a drain region formed from atleast a first type of impurity; and a second transistor, formed fromsaid thin silicon film, and carried by the substrate, said secondtransistor having a source region and a drain region formed from atleast said first type of impurity and a second type of impurity; whereinthe concentrations of the first type of impurity and second type ofimpurity are different from each other.
 2. The CMOS device of claim 1,wherein the thin film transistor is a P-type having source and drainregions formed from at least acceptor impurities and the secondtransistor is an N-type having source and drain regions formed from atleast donor impurities, the concentration of donor impurities in thesecond transistor being greater than the concentration of acceptorimpurities in the thin film transistor.
 3. The CMOS device of claim 1,wherein the thin film transistor is an N-type having source and drainregions formed from at least donor impurities and the second transistoris a P-type having source and drain regions formed from at leastacceptor impurities, the concentration of acceptor impurities in thesecond transistor being greater than the concentration of donorimpurities in the thin film transistor.